Anyone know about digestion 'cleanses'?

A guy at work recently suggested this, saying it will boost my nutrition intake by cleaning out my digestive tract, also minor weight loss from the shedding of impurities. But he's just some guy at work, not a fitness expert. I do remember hearing about this on the radio awhile back. It sounds like a sensible enough concept, cleaning out the intestines so they can better absorb nutrients, but before I go throw my money at GNC, I want to know if anyone has some insight into how effective this can be. Help a fellow out?